The invention relates to a method for diagnosing a NOx readings recorder which acquires a NOx concentration in an exhaust gas tract of an internal combustion engine and comprises two measuring cells, wherein the exhaust gas to be measured is supplied to the first measuring cell and an oxygen concentration is set by means of a first oxygen ion pump current, wherein the second measuring cell is connected to said first measuring cell and wherein both measuring cells are disposed in a solid electrolyte.
The invention also further relates to a computer program and a computer program product, which are suitable for carrying out the method.
In order to reduce the fuel consumption of motor vehicles, internal combustion engines are increasingly being operated with a lean fuel/air mixture. The efficiency of the internal combustion engine is particularly high in this so-called lean operation. In order to meet the stringent exhaust emission limits, a particular exhaust gas aftertreatment is then required, which especially allows for a reduction of nitrogen oxides (NOx) in the exhaust gas.
For this reason, so-called NOx storage catalytic converters are used, which are capable of adsorbing NOx compounds that arise during lean combustion out of the exhaust gas. An “evacuation” of such a NOx catalytic converter is necessary from time to time. Evacuation thereby means as much as conversion of NOx compounds to harmless compounds. In so doing, carbon monoxide, hydrogen and hydrocarbon are, for example, worth considering as reducing agents. A NOx sensor, respectively NOx readings recorder, is disposed downstream of the NOx storage catalytic converter in the exhaust gas duct for the determination of the degree of loading of said NOx storage catalytic converter and for the initiation of the regeneration of said NOx storage catalytic converter.
As an alternative to the exhaust gas aftertreatment by means of NOx storage catalytic converters, so-called selective catalytic reaction systems can be used. A NOx readings recorder is likewise used for the control of said systems.
A NOx readings recorder of this kind is known, for example, from the American patent publication U.S. Pat. No. 6,290,829 B1.
A NOx readings recorder of this kind constitutes an exhaust emission-relevant part. Exhaust emission-relevant parts must be continually and constantly checked for their operability. Such checks are required by law in both Europe and America. In the future, monitoring limit values will have to be complied with for monitoring NOx emissions.
In order to meet this objective, it is necessary within the scope of the on-board diagnostics (OBD) to monitor all components which serve to reduce nitrogen oxides in motor vehicles for the operability thereof. Hence a faulty NOx readings recorder from model year 2009 will, for example, have to be detected as faulty when a value 3.5 times that of the limit value is exceeded, from model year 2010 when a value 2.5 times that of the limit value is exceeded and from model year 2013 when a value 1.5 times that of the limit value is exceeded.
Only serious errors of the NOx sensor can be recognized using current diagnostics known per se.
Moreover, a monitoring of a characteristic curve drift in vehicles with diesel internal combustion engines has not been possible up until now because on the one hand no NOx emissions are present in the test cycle after a selective catalytic reduction (SCR—selective catalytic reduction—Nox reduction catalytic converter with urea injection). On the other hand, it is virtually impossible in diesel systems of this type to set exact and known NOx concentrations by means of engine-related measures. In such a system, a reference NOx value cannot be set and is not available.
A plausibility check of the communication of the NOx sensors between themselves is likewise not possible because this would entail an active conversion reduction for a long period of time, which would result in the SCR storage system having to be run empty. This in turn would lead to the NOx emissions being significantly increased at times during the test cycle and driving operation.
In the case that NOx sensors are present, so-called internal “self-diagnoses” are known from the technical field for gasoline operated engines under stoichiometric conditions (8=1). These self-diagnoses are, for example, described in the German patent publication DE 103 12 732 B4 as well as in the German patent publication DE 10 2006 053 841A1. An application at 8=1 is, however, extremely difficult to carry out in diesel systems. In other words, conditions for stability (e.g. a stable 8) required for a successful self-diagnosis are not feasible. Additional disadvantages of an internal self-diagnosis of this type are:                complex interventions into the pump current regulator of the NOx sensor are required;        very stable exhaust gas conditions are required, which often are not feasible when the vehicle is in operation;        as a result of a targeted setting of an O2 partial pressure in the second chamber of the NOx readings recorder and comparison with a reference value, effects of drift and ageing on the pump electrodes can distort the outcome.        
The aim of the invention is therefore to convey a method for diagnosing a NOx readings recorder, particularly for recognizing the presence of a characteristic curve drift of such a NOx sensor, in diesel vehicles which delivers a diagnostic result at various operating points during driving operation as well as within restricted test cycles; thus enabling statutory exhaust emission limits to be met.